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recent studies or overviews have also tended to focus on
the river types in these drylands (e.g. Cohen and Laronne,
2005; Hassan, 2005; Thomas, 2005; Powell
et al.
, 2007;
Pollen-Bankhead
et al.
, 2009), in many cases unwittingly
reinforcing the impression that these characteristics apply
to most or all dryland rivers. This poses some important,
interrelated questions. To what extent does the climatic
setting actually impart a distinctive character to dryland
rivers? Do exogenous or endogenous rivers with perennial
flows (e.g. as fed by snowmelt or groundwater resurgence)
exhibit characteristics more akin to humid zone perennial
rivers? Why are many rivers in Australia so different from
many rivers in other drylands? To what extent can we actu-
ally generalise about dryland river characteristics? Such
questions highlight the need for a global assessment of
dryland rivers that explains their diversity and identifies
their distinctiveness.
terms of a linear flow occupancy scale. The location of
any given dryland river on that scale influences the nature
of hydrologic inputs, throughputs and outputs, as well
as fluvial network and channel characteristics. Similarly,
other natural environmental factors - tectonic and struc-
tural setting, lithology, vegetation - can influence dryland
rivers in diverse ways (Table 12.2). In addition, different
drylands have experienced different environmental (cli-
matic, tectonic, vegetative) histories over Cenozoic, his-
torical and instrumental timescales (Chapters 3 and 4).
Although the details of these past and present changes
are still being deciphered, across the global extent of dry-
lands, these different combinations of environmental fac-
tors, environmental changes and catchment scales might
be expected to lead to great diversity in dryland river
characteristics.
'River style' (more broadly, 'fluvial style') is a term
that is generally used to refer to the overall geomorpho-
logical and sedimentary character of a river, including
cross-sectional, planform/pattern and longitudinal profile
features. A first-order global assessment of dryland rivers
should therefore focus on the spatial extent and frequency
of occurrence of different styles, but this cannot yet be
done comprehensively. As mentioned above, large parts
of many drylands remain relatively poorly known in flu-
vial terms, including large areas of Australia and south-
ern Africa, but even less information is known about river
characteristics in the extensive South American and Asian
drylands. Within the last few years, the release of virtual
globes such as Google Earth (Tooth, 2006) have opened up
possibilities of undertaking preliminary planform-based
assessments of dryland river characteristics, but this has
yet to be done systematically for the world's drylands as
a whole, and certainly has not been matched by a corre-
sponding increase in field-based investigations.
Until such comprehensive global assessments are un-
dertaken, a comparative regional assessment is the best
approach, as this will at least demonstrate how different
combinations of factors can give rise to different dryland
river styles. Consequently, the following sections examine
river styles in three different drylands that present a spec-
trum of environmental conditions (Table 12.3). One region
is the Mediterranean, which is taken to be broadly repre-
sentative of the characteristics of relatively high energy
rivers draining predominantly small, steep catchments
with varying degrees of tectonic activity (Figure 12.4(a)).
The other regions offer significantly different perspec-
tives. Southern Africa is taken to be more representa-
tive of moderate to low energy rivers draining dominantly
moderate-size, moderate-gradient, tectonically quiescent
catchments (Figure 12.2(a) and (b)). Australia is examined
12.3
Diversity of dryland rivers
A useful starting point for assessing dryland river diver-
sity is to highlight the fact that river systems are the inte-
grated product of their environmental settings, including
climatic, tectonic, structural, lithological and vegetative
factors. Through their influence on gross physiography,
runoff and sediment supply, these environmental factors
impact directly and indirectly on river process, form and
behaviour. In addition, none of these factors are constant
but instead are subject to various degrees of change on
different spatial and temporal scales: climate can vary
through time, tectonic activity can wax and wane, dif-
ferent lithologies can be uncovered during landscape de-
nudation, vegetation cover can fluctuate in density, com-
position or health, and so on. River characteristics are in
part a reflection of their environmental histories (possi-
bly including inheritance from more humid conditions)
but they can also be impacted more-or-less continually by
ongoing environmental changes, with geomorphic setting
and catchment scale being important determinants of how
different rivers will respond to changing conditions (e.g.
Nanson and Tooth, 1999).
Drylands are characterised by varied degrees of arid-
ity and exist across a wide range of tectonic, structural,
lithological and vegetative settings (Chapters 1 and 2). It
has been suggested, for example, that drylands encom-
pass a higher diversity of hydrological conditions than
more humid zones (e.g. Pilgrim, Chapman and Doran,
1988; Knighton and Nanson, 1997; Nanson, Tooth and
Knighton, 2002). This is exemplified by Figure 12.3,
which highlights the fact that dryland rivers exist across
